1. Field of the Invention
The present invention relates to a thermoplastic resin composition containing inorganic fibers as reinforcement. More particularly, it relates to a polyamide resin composition having high strength and high modulus of elasticity.
2. Description of the Prior Art
As the plastics have recently begun to be used for making machine elements such as gears and structures, there has arisen a demand for plastics materials having high strength and high modulus of elasticity. One way of meeting such a demand is incorporating thermoplastic resins with glass fiber, carbon fiber, potassium titanate whisker, and the like, individually or in combination with one another. It is known that when glass fiber is combined with carbon fiber, talc or calcium carbonate in the form of powder or flake, or potassium titanate whisker, the resulting resin composition has very high strength and high modulus of elasticity. Such combination of reinforcements has disadvantages. That is, carbon fiber and potassium titanate whisker are expensive, and talc and calcium carbonate in the form of powder or flake impede reinforcement by glass fiber. Moreover glass fiber deteriorates the surface of moldings when used in large quantities.
On the other hand, there is another known reinforcement for thermoplastic resins. It is ground mineral fiber having comparatively short length and controlled range of aspect ratios (ratio of length to diameter of a fiber). Typical ground mineral fiber is shown in the following U.S. Pat. No. 4,124,730, and Japanese Patent Publication (unexamined) Nos. 51155/82, 90036/82, 52335/83 and 52341/83. The thermoplastic resin compositions disclosed in these patents are not necessary able to meet the demand for high strength and high modulus of elasticity.
In order to overcome the above-mentioned disadvantages and to produce an inexpensive resin composition which has high strength and high modulus of elasticity and provides good surfaces of moldings, the present inventors carried out a series of researches. As the result, it was found that a polyamide resin filled with glass fiber and ground mineral fiber having an aspect ratio of 5 to 100 in certain quantities is more improved in strength as compared with the one filled with glass fiber alone. The combination of the two reinforcements saves the comparatively expensive glass fiber and polyamide resin. Moreover, the resin composition of this invention provides good surfaces of moldings. The present invention was completed based on these findings.